Fix crash if /etc/tinydm.d/env-wayland.d dir exists (as created by
Makefile) but is empty:

  tinydm: sourcing file: /etc/tinydm.d/env-wayland.d/*
  /usr/bin/tinydm-run-session: .: line 38: can't open '/etc/tinydm.d/env-wayland.d/*': No such file or directory

Fixes: a7b0eaf3 ("Add support for sourcing env profiles based on session type")